1. Field of the Invention
The present invention relates to an image forming apparatus.
2. Description of Related Art
In general, an electrophotographic image forming apparatus (such as a printer, a copy machine, and a fax machine) is configured to irradiate (expose) a charged photoconductor with (to) laser light based on image data to form an electrostatic latent image on the surface of the photoconductor. The electrostatic latent image is then visualized by supplying toner from a developing device to the photoconductor on which the electrostatic latent image is formed, whereby a toner image is formed. Further, the toner image is directly or indirectly transferred to a sheet, and then heat and pressure are applied to the sheet at a fixing nip to form a toner image on the sheet.
Conventionally, in the above-mentioned image forming apparatus, when thick paper having a relatively large thickness is used as a sheet, linear density unevenness that is called shock jitter has been caused in some cases. Such density unevenness is caused when the on the driving source of an image bearing member is abruptly increased and the surface movement velocity of the image bearing member is largely and momentarily increased at the time when thick paper enters a transfer position (for example, a secondary transfer nip) where the image bearing member (for example, an intermediate transfer belt) that rotates while bearing a toner image and a transfer member (for example, a secondary transfer roller) that rotates while making contact with the image bearing member and transfers the toner image formed on the surface of the image bearing member to a sheet make contact with each other.
Japanese Patent Application Laid-Open No. 2009-198596 discloses a technique for reducing shock jitter and transfer defect which can be caused when the distance between the surface of the intermediate transfer belt and the rotational axis of the secondary transfer roller falls outside a proper distance due to the change of the diameter and the elastic modulus of the secondary transfer roller. In the technique disclosed in Japanese Patent Application Laid-Open No. 2009-198596, a thickness sensor configured to detect the thickness of a sheet (recording sheet) and a distance sensor configured to detect the position of the secondary transfer roller are provided, and the position of the secondary transfer roller in the state where an eccentric cam is in contact with a swing arm is adjusted on the basis of a detection result obtained by the thickness sensor and a detection result obtained by the distance sensor in the state where the eccentric cam is not in contact with the swing arm configured to hold the secondary transfer roller in a swingable manner.
The technique disclosed in Japanese Patent Application Laid-Open No. 2009-198596 includes a mechanism configured to form a secondary transfer nip effective for suppressing shock jitter by controlling the center distance between the secondary transfer roller and a transfer counter roller that faces the secondary transfer roller with the intermediate transfer belt therebetween in accordance with the thickness of the sheet. To be more specific, the secondary transfer roller in synchronization with the swing member is brought into contact with the transfer counter roller with the spring load of a pressing spring, and a stabilized position (that is, a position where an appropriate transfer nip pressure is obtained) is set as a reference position, and, the swing member is pushed down by the eccentric cam by a distance corresponding to the thickness of the sheet while utilizing a result of detection of the distance sensor. In such a mechanism, the secondary transfer nip is formed with the spring load, and therefore, for the purpose of minimizing the variation of the spring load due to displacement of the pressing spring at the time of entering and leaving of the sheet (at the time when the sheet enters the secondary transfer nip, and when the sheet leaves the secondary transfer nip), the elasticity coefficient of the pressing spring (difficulty of deformation) is set to a significantly small value. However, in the case where the secondary transfer roller is composed of a roller having a certain mass such as a hard roller for example, the position of the secondary transfer roller is displaced at the time of entering and leaving of the sheet, and acceleration is generated at the secondary transfer roller. As such, the above-mentioned mechanism in which the pressing spring has a significantly small elasticity coefficient behaves as if the secondary transfer roller bounds. As a result, the load on the driving source of the intermediate transfer belt may be abruptly increased, and the surface movement velocity of the intermediate transfer belt may be momentarily reduced, thus generating shock jitter.